Copyright © 2015 the American Physiological Society. Flow-mediated dilation (FMD) is a noninvasive technique used to measure conduit artery vascular function. Limited information is available on normative FMD values in healthy children and adolescents. The objective of this study was to assess relationships between age and sex with FMD across childhood and adolescence. Nine hundred and seventy-eight asymptomatic children (12 ± 3 yr, range 6-18 yr, 530 male) underwent ultrasonic brachial artery assessment before and after 5 min of forearm ischemia. Sex differences in FMD and baseline artery diameter were assessed using mixed linear models. Baseline artery diameter was smaller in females than males [2.96 mm (95% CI: 2.92-3.00) vs. 3.24 mm (3.19-3.28), P <0.001] and increased with age across the cohort (P <0.001). Diameter increased between ages 6 and 17 yr in males [from 2.81 mm (2.63, 3.00) to 3.91 mm (3.68, 4.14)] but plateaued at age 12 yr in females. Males had a lower FMD [7.62% (7.33-7.91) vs. 8.31% (7.95-8.66), P = 0.024], specifically at ages 17 and 18 yr. There was a significant effect of age on FMD (P = 0.023), with a reduction in FMD apparent postpuberty in males. In conclusion, the brachial artery increases structurally with age in both sexes; however, there are sex differences in the timing and rate of growth, in line with typical sex-specific adolescent growth patterns. Males have a lower FMD than females, and FMD appears to decline with age; however, these findings are driven by reductions in FMD as males near maturity. The use of age- and sex-specific FMD data may therefore not be pertinent in childhood and adolescence.